Process for polishing crystals



United States Patent 3,321,872 PROCESS FOR PULISHENG CRYSTALS George A. Lindstrom, Griffith, Ind., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed Aug. 3, 1964, Ser. No. 387,197 9 Claims. (Cl. l283) This invention relates to a process for polishing crystals and, in particular, to a process for polishing alphaaluminum oxide crystals such as ruby or sapphire.

Synthetic crystals of ruby, for example, are fabricated into rods for use in laser devices. Such crystal rods must have ends which are polished very flat and which are also parallel. Polishing of such materials generally consists of rubbing the face which is to be finished flat on a flat lapping surface charged with abrasives. When used in laser applications, the end surfaces of the crystal rod should have an optical finish and a flatness approaching onemillionth of an inch in order to allow the intense light beam to be transmitted with minimum of scattering. Achieving such flatnesses by polishing has been a problem for the currently used lapping materials tend to leave scratches on the crystal surface.

It is the object of this invention to provide an improved method for polishing such materials whereby very flat faces may be produced.

It is a further object of this invention to provide a process for polishing the ends of ruby rods to an optical finish and flatness approaching a millionth of an inch.

Other aims and advantages of this invention will be apparent in view of the following disclosure and the appended claims.

In accordance with this invention a process for polishing crystal bodies is provided comprising rough lapping the surface of the crystal to be polished on a flat aluminum surface covered with finely divided diamond abrasive compound, and finishing the polishing of the crystal surface by then lapping the surface on a fiat tin surface covered with a liner diamond abrasive compound.

It has been found that the use of aluminum lapping plates in the rough polishing of a crystal body followed by the use of tin lapping plate gives an optical finish and the desired flatness to the crystal. This is particularly so in the case of alumina crystals of ruby or sapphire Such materials may be polished to an optical finish and flatness approaching a millionth of an inch using the process of this invention.

Various other materials were tested for use as lapping plates for the polishing of alumina-based crystals such as ruby. For example, in the roughing process, that is the process in which the largest scratches are removed, materials including quartz, brass, bronze, Meehanite, glass, and various plastics were used. The use of these materials resulted in a finish on the ruby that contained scratches and other imperfections. The aluminum lapping plate was found to give the best finish in regard to the roughing operation.

In regard to the final polishing step, it was found that the use of a tin lapping plate after the roughing on the aluminum plates was the best material. In addition to aluminum itself, the following materials were tested for suitability as the final lapping plate: lead, 70 percent lead-30 percent tin alloy, Babbitt metal, and printers metal. With all of these materials, the finally polished surface was found to be only slightly improved over the roughly polished surface. The only successful combination of lapping materials for achieving the optical finish and flatness described herein is the use of aluminum roughing plates followed by a tin finishing plate. It is to be noted that these two materials must be used in conjunction with each other and in the indicated sequence if the desired surface flatness is to be achieved. The use of either material alone or in conjunction with other materials will not give the superior surfaces achievable with the use of the aluminum-tin plates.

The aluminum and tin lapping plates should themselves be ground and lapped to achieve a 20 to 30 micron finish. The aluminum and tin lapping plates should be constructed from high purity soft metals and should have a low porosity. These plates should be polished to the indicated flatness using a 500 mesh non-charging corundurn powder. It is an important requirement of this invention that the aluminum and tin lapping surfaces be constructed of high purity, soft and non-porous aluminum and tin stock. The use of these soft forms of the metals insures that scratches will not be transmitted from the lapping plates to the crystal surface.

In the roughing operation, a series of aluminum lapping plates are used with a series of increasingly finer diamond abrasive compounds to bring the crystal surface down to a near optical finish which is then followed by the final polish with a tin lapping plate and a finer diamond abrasive compound.

The diamond abrasive compounds are generally paste like mixtures of the indicated particle size of diamond dust in a suspension vehicle. These diamond abrasive compounds are generally commercially available in the grades used herein. An oil-soluble lubricant is preferably used along with the diamond abrasive compound on the lapping plates.

In the practice of this invention, a ruby rod could be fabricated from a synthetic unicrystalline ruby boule grown by the flame fusion process or from a ruby crystal body grown by any other process. The ruby rod is a cylinder whose ends must be ground flat as near parallel as possible. The roughing operation is commenced using an aluminum lapping plate as described above which has been itself polished flat to about a 20 to 30 micron finish. A 30 to 40 micron diamond abrasive compound is used on the lapping plate and an end surface of the crystal rod is rubbed over the lapping plate. When done by hand in a circular motion, about five minutes of rubbing is used. This step, and in fact, all steps of the process could be performed on an optical machine wherein one or more crystal rods are secured and supported in a block so as to be perpendicular to a rotating lapping plate on which the block rides. The block is oscillated so that the crystal surfaces do not rub against the lapping plate in the same path.

In the next step another aluminum lapping plate is used with a 6 micron diamond abrasive compound. If done manually for about 5 minutes, this would remove all of the 30 micron scratches from the crystal surface.

In the final stage of the roughing operation another aluminum plate is used with a micron diamond abrasive compound. An optical machine should be used here to rotate and oscillate the crystal. Using a machine it should take about 2 hours to bring the flatness down to near the finish required. The lapping plate is preferably lubricated prior to these final stages with very fine noncharging corundum powder. During the lapping operation an oil soluble lubricant is also used with the diamond abrasive compound on the lapping plates.

In the finishing operation, the tin lapping plate is used. This lapping plate is polished as indicated previously to a flatness of about 20 to 30 microns and preferably lubricated with very fine (about 15 microns) non-charging corundum powder before the crystal polishing operation is started. The flat tin lapping surface is covered with a micron diamond abrasive compound. The roughly polished crystal is polished on this surface in a machine until the desired flatness is achieved. This could take from /2 hour to 4 hours.

Variations in the first three steps, the rough polishing operation, may be made within the scope of this invention. For example, the first two steps could be accomplished with the aid of a grinding machine, especially if many crystals are to be polished. A number of crystal rods could be supported in a block and ground together. Following this the crystals could be lapped on the aluminum plate using a 3 micron diamond abrasive compound. The final polishing step is then completed with the tin lapping surface.

Using the procedure of the four step operation outlined above a ruby crystal was polished on both ends to a flatness of a millionth of an inch. The flatness is measured by means of an interferometer wherein an optical flat of known flatness is used as a standard and interference fringes from the crystal surface are viewed for parallelism. After one end of a crystal rod has been polished, either partially or completely, the block holding the crystal rod may be reversed and the other end polished. Using special instruments, this surface is lapped parallel to the other end. Using the polishing process of this invention, it is possible to parallel the ends of a ruby crystal rod to within 1 second of arc to each other.

The process of this invention has been described in regard to the polishing of alpha-alumina crystals (also referred to herein as alumina crystals) and in particular, ruby crystals. The process, however, is also applicable to the polishing of other types of crystals.

What is claimed is:

1. A process for polishing a surface of a crystal comprising rubbing said crystal surface on an aluminum lapping surface covered with a finely divided diamond abrasive compound to polish said crystal surface to a near flatness, and thereafter rubbing said crystal surface on a tin lapping surface covered with a finer diamond abrasive compound to achieve a more nearly flat surface.

2. A process for polishing a surface of a crystal comprising rubbing said crystal surface on an aluminum lapping surface covered with a finely divided diamond abrasive compound, then rubbing said partially polished crystal surface on at least one other aluminum lapping surface covered with successively finer diamond abrasive compounds to polish said crystal surface to a near flatness, and then rubbing said crystal surface on a tin lapping surface covered with a still finer diamond abrasive compound to achieve a more nearly flat surface.

3. A process for polishing a surface of a crystal comprising rubbing said crystal surface on an aluminum lapping surface covered with a 30 to 40 micron particle size diamond abrasive compound to polish said crystal surface, then rubbing said crystal surface on an aluminum lapping surface covered with a 6 micron diamond abrasive compound and then rubbing said crystal surface on an aluminum lapping surface covered with a A micron diamond abrasive compound to achieve a near flatness on said crystal surface, and thereafter rubbing said crystal surface on a tin lapping surface covered with a micron diamond abrasive compound to achieve a more nearly flat surface.

4. The process in accordance with claim 3 wherein the lapping surfaces covered with diamond abrasive compounds are also lubricated with an oil-soluble lubricant during the crystal polishing.

5. The process in accordance with claim 3 wherein the aluminum and tin lapping surfaces are each composed of soft, high-purity, non-porous metal.

6. The process in accordance with claim 5 wherein the aluminum and tin lapping surfaces were each polished with finely divided corundum to a flatness of about 20 to 30 microns prior to the crystal polishing operation.

7. A process for polishing a surface of an aluminabased crystal comprising rubbing the surface of said crystal to be polished on an aluminum lapping surface covered with a 30 to 40 micron particle size diamond abrasive compound to polish said crystal surface, then rubbing said crystal surface on an aluminum lapping surface covered with a 6 micron diamond abrasive compound and then rubbing said crystal surface on a aluminum lapping surface covered with a A micron diamond abrasive compound to achieve a near flatness on said crystal surface, and thereafter rubbing said crystal surface on a tin lapping surface covered with a micron diamond abrasive compound to achieve a more nearly flat surface.

8. A process for polishing the ends of a synthetic, unicrystalline, cylindrical ruby rod to make said ends flat and parallel, comprising rubbing one of said ends on an aluminum lapping surface covered with a 30 to 40 micron particle size diamond abrasive compound, then rubbing said crystal end on at least one other aluminum lapping surface covered with successively finer diamond abrasive compounds and finishing with a diamond abrasive compound of about micron particle size to polish said crystal end to a near flatness, each aluminum lapping surface being composed of a soft, high purity, non-porous aluminum metal and having been previously polished to a flatness of about 20 to 30 microns, each rubbing operation being performed with an oil soluble lubricant on said lapping surfaces, and thereafter rubbing the partially polished crystal end on a tin lapping surface, said surface composed of soft, high-purity, non-porous tin and previously polished to a flatness of about 20 to 30 microns, said surface covered with a ,5 micron diamond abrasive compound with an oil-soluble lubricant on said lapping surface to achieve a more nearly flat surface on said crystal end, and then polishing the other end of said rod in the same manner while maintaining said end being polished parallel to the end previously polished.

9. A process in accordance with claim 3 wherein the crystal to be polished is an alumina-based crystal.

References Cited UNITED STATES PATENTS 2,719,390 10/1955 Jones 51283 X 3,177,628 4/1965 Highberg 5l-283 LESTER M. SWINGLE, Primary Examiner. 

1. A PROCESS FOR POLISHING A SURFACE OF A CRYSTAL COMPRISING RUBBING SAID CRYSTAL SURFACE ONAN ALUMINUM LAPPING SURFACE COVERED WITH A FINELY DIVIDED DIAMOND ABRASIVE COMPOUND TO POLISH SAID CRYSTAL SURFACE TO A NEAR FLATNESS, AND THEREAFTER RUBBING SAID CRYSTAL SURFACE ON A TIN LAPPING SURFACE COVERED WITH A FINER DIAMOND ABRASIVE COMPOUND TO ACHIEVE A MORE NEARLY FLAT SURFACE. 